rtnetlink: Fix message size calculation for link messages

nlmsg_total_size() calculates the length of a netlink message
including header and alignment. nla_total_size() calculates the
space an individual attribute consumes which was meant to be used
in this context.

Also, ensure to account for the attribute header for the
IFLA_INFO_XSTATS attribute as implementations of get_xstats_size()
seem to assume that we do so.

The addition of two message headers minus the missing attribute
header resulted in a calculated message size that was larger than
required. Therefore we never risked running out of skb tailroom.
Signed-off-by: NThomas Graf <tgraf@infradead.org>
Acked-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -347,16 +347,17 @@ static size_t rtnl_link_get_size(const struct net_device *dev)
 	if (!ops)
 		return 0;
 
-	size = nlmsg_total_size(sizeof(struct nlattr)) + /* IFLA_LINKINFO */
-	       nlmsg_total_size(strlen(ops->kind) + 1);	 /* IFLA_INFO_KIND */
+	size = nla_total_size(sizeof(struct nlattr)) + /* IFLA_LINKINFO */
+	       nla_total_size(strlen(ops->kind) + 1);  /* IFLA_INFO_KIND */
 
 	if (ops->get_size)
 		/* IFLA_INFO_DATA + nested data */
-		size += nlmsg_total_size(sizeof(struct nlattr)) +
+		size += nla_total_size(sizeof(struct nlattr)) +
 			ops->get_size(dev);
 
 	if (ops->get_xstats_size)
-		size += ops->get_xstats_size(dev);	/* IFLA_INFO_XSTATS */
+		/* IFLA_INFO_XSTATS */
+		size += nla_total_size(ops->get_xstats_size(dev));
 
 	return size;
 }